The liquid machining medium supplied to the EDM gap is typically a liquid hydrocarbon such as kerosine or a conductivity-adjusted water liquid. It serves to envelope, compress and cool the discharge column. By causing a discharge column to be squeezed to a greater extent by the liquid environment, it is possible to increase the discharge current density and discharge energy density, and to augment the discharge pressure, thereby increasing the impulsive pulverization and scattering of molten metal at the discharge spot and metal vapors in the discharge column. The EDM liquid machining medium also serves to cool these discharge scattered products and thereby to solidify them into particulate objects which are commonly called EDM chips. The EDM liquid machining medium further serves to carry away the discharge products and EDM chips from the discharge site. Thus, a portion of the liquid medium is decomposed and evaporated to produce a gaseous expansion pressure which facilitates removal of the machining products from the region of the machining gap between the tool electrode and the workpiece.
I have discovered that upon extinction of one given electrical discharge, a portion of the discharge-generated gases and vapors should desirably remain in the gap region to facilitate triggering of a next discharge. Thus, a succession of outstandingly effective electrical discharges will be created across the machining gap between the tool electrode and the workpiece if an appropriate amount of these gases and vapors is kept in existence in the gap. An excessive amount of the gases and vapors will be undesirable because the discharges tend to be converted into a continuous arc discharge which brings about damage of the workpiece, the tool electrode or both. At the same time, the cooling action will be substantially deteriorated. On the other hand, instability in discharge production or a reduced rate of repetition of electrical discharges is attributed to an insufficient proportion of the discharge-generated gases and vapors.